Originally Posted by Starting Strength, 3rd ed., p48-52
Since the adductors tend to pull the knees in, what keeps them out when you use your hips correctly? If ad-duction, of the thighs means pulling the distsal end of the femurs (the knees) toward the midline of the body, it seems like ab-duction would be the movement used to keep the knees out, and that the abductors would be the muscles that did this. But the abductors consist of only the tensor fascia latae (TFL, a small muscle that connects the hip at the anterior iliac crest to the lower leg), the gluteus medius, and the gluteus minimus. Together they create hip abduction if you raise your leg out to the side, away from your body. Since nobody actually does this, except to demonstrate the definition of abduction in biomechanics class, this is probably not what is going on when we squat.
External rotation occurs when you make your right femur rotate clockwise and your left femur rotate counterclockwise, as when you stand up and pivot on your heels to rotate your toes away from each other. There are at least nine muscles that perform this function: the gluteus medius, minimus, and maximus, the adductor minimus, the quadratus femoris, the inferior gemellus, the obturator internus, the superior gemellus, and the piriformis. (Notice that the external rotators include two of the abductor muscles.) External rotation is critical to stabilizing gait mechanics through the stride. As it relates to our analysis, the action of rotating the femurs out is what actually occurs when you shove your knees out on the way down to the bottom of the squat. Prove this to yourself by sitting in a chair and rotating your femurs the same way you would if you were standing up and pivoting on your heels to point your toes out. Using the external rotators to set the knees in a position parallel to the feet makes all kinds of sense when you consider that they are in an effective position to do it and the TFL is not. So shoving the knees out at the top of the squat, and keeping them there so that the adductors can do their job, is accomplished by the muscles that rotate the hips externally. These muscles anchor the thigh position that allows for both good squat depth and the more effective use of all the muscles of the hips.
When you intentionally shove your knees to the outside as you come down into the bottom of the squat, not only do you get the femurs away from the ASIS and the gut, but you also allow the adductors to stretch tighter and position themselves to more effectively contract as they reach the limit of their extensibility. A tight, stretched muscle contracts harder than a looser, shorter muscle does because the stretch tells the neuromuscular system that a contraction is about to follow. A more efficient firing of the more contractile units always happens when preceded by a stretch....
... If you allow your knees to come together at any time during the squat, you dilute the function of the muscles both medial and lateral to the femurs. But this problem cannot be corrected if it is not identified. When you squat, look down even more than usual, to a point on the floor right between your toes, where you can clearly see your knees, and check your position. If your knees move toward each other at any point during the squat, shove them out. You will probably have to exaggerate this shoving-out in order for it to put your knees in the correct position, since you thought they were in the right position when they were coming in. When you get them back out to parallel with your feet and keep them there for a couple of sets, you will notice later that your adductors, and perhaps your most lateral glutes, get sore. From our previous discussion, you know why.